1. Field of the Invention
The present invention relates to a driving torque control apparatus for a four-wheel drive vehicle which controls driving torque applied to each wheel of the four-wheel drive vehicle, and a driving torque control method performed therein.
2. Related Arts
Conventionally, four-wheel drive vehicles which can move on a bad condition road such as an unpaved road or a snowy road by transmitting driving torque generated by an engine to four wheels is put to practical use.
In the conventional four-wheel drive vehicle, if one of four wheels spins, driving torque can not be transmitted from the engine to the other wheels. As a result, the vehicle can not have enough tractive power to advance forward.
In other words, in the conventional four-wheel drive vehicle, driving torque from the engine is distributed to drive shafts for front and rear wheels through a center differential gear, driving torque distributed to the front wheel drive shaft is further distributed to front-left and front-right wheels through a front differential gear and driving torque distributed to the rear wheel drive shaft is further distributed to rear-left and rear-right wheels through a rear differential gear. Therefore, if any one of wheels spins, driving torque is not transmitted to the other wheel due to the action of each of the differential gears.
As a technique to solve such a problem, Japanese Patent Application Laid-Open No. (JP-A-) 60-248440 discloses an apparatus which detects a slippage of each of wheels and provides braking torque to a wheel falling in a spinning state. According to this technique, wheels are prevented from spinning, whereby driving torque is transmitted to non-spinning wheels even when one of wheels spins. That is, driving torque transmitted to the spinning wheel is reduced by applying braking torque thereto. As a result, a rotational speed differential among a plurality of wheels is restrained.
However, because the above-described apparatus merely provides braking torque to the spinning wheel (i.e., reduces driving torque thereto), running stability of the vehicle can be deteriorated.
For example, in a situation that either one of front-left and front-right wheels spins and a rotational speed differential between rear-left and rear-right wheels is being generated, if braking torque is provided to the spinning wheel (a front-left wheel or a front-right wheel), driving torque transmitted to the rear wheels is increased due to the action of the respective differential gears, with the result that the rotational speed differential between the rear-left and rear-right wheels is made larger. As a result, behavior of the vehicle is apt to over-steer, thereby deteriorating running stability of the vehicle.
Further, the above-described apparatus controls braking torque only when one of four wheels falls into a spinning state. That is, JP-A-60-248440 fails to teach driving torque control which is effective when all of four wheels simultaneously fall into the spinning state. Furthermore, because the above-described apparatus determines a spinning wheel based on a rotational speed differential between the respective wheels, if the four wheels are spinning to the same extent, the apparatus cannot detect spinning state of each of four wheels. Therefore, if the vehicle starts to travel on, for example, a slippery road having a low friction coefficient and four wheels start to spin at the same time and to the same extent, no control is performed to the spinning wheels, thereby deteriorating running stability of the vehicle.